For vaginal transmission, HIV must penetrate mucus secretions to reach target cells; we recently found that HIV readily diffuses through pH-neutralized human cervicovaginal mucus. Most antibodies produced by the immune system are secreted into mucus (not blood or lymph), and when IgG is applied topically it provides robust protection against HIV at mucosal surfaces. However, how secreted or topically applied IgG protects against infections in mucus remains poorly understood. We hypothesize that in addition to well-known antibody functions (e.g., neutralization, complement activation, opsonization), an unrecognized effector function of IgG is to trap viruses in mucus. Hypothesis: By forming weak, short-lived affinity bonds with the mucus gel, IgG is still free to diffuse through mucus and quickly bind to viruses. As IgG accumulates on the virus surface, the array of IgGs may form multiple low-affinity adhesive crosslinks between the virus and the mucus gel. A sufficient number of these low-affinity crosslinks, possibly at sub-neutralizing IgG concentrations, may permanently trap the virus in the mucus gel. Trapping reduces the flux of virus that reaches target cells, and facilitates inactivation and clearance by additional protective mechanisms. This potential IgG trapping function in mucus has been largely unrecognized because most studies of IgG activity have not been performed in mucus. In this proposal, we seek to test the hypothesis that anti-HIV IgGs will trap HIV in mucus. Our pilot observations indicate that remarkably low concentrations of specific IgG1can trap Herpes Simplex Virus and virus-like particles that otherwise rapidly penetrate mucus gels. Trapping HIV in mucus will likely reduce or block any effective contact between HIV and target cells. Trapped HIV will be shed with postcoital discharge, and/or become inactivated by thermal degradation, defensins, and other protective mechanisms. If confirmed for HIV, IgG-induced trapping in mucus may help explain mucosal protection at sub-neutralizing concentrations, and perhaps also help explain the modest protection observed in the recent RV144 Thai HIV- vaccine trial. Aim 1: Using high resolution particle tracking, observe diffusion rates of hundreds of HIV virions (with internal GFP tag) in neutralized human CVM treated with anti-HIV IgGs, including broadly neutralizing b12, 2G12, 2F5 and 4E10 as well as non-neutralizing A32 and 50-69. Quantify HIV diffusion (i.e. trapping efficiency) as a function of IgG concentration and number of IgG molecules bound per HIV virion. Aim 2: Measure diffusion rates of HIV in seminal plasma mixed with human CVM treated with anti-HIV IgGs, as well as in seminal plasma alone treated with anti-HIV-IgGs. Aim 3: Determine if anti-HIV IgG in mucus can block HIV virions from reaching and binding to heparan-sulfate coated surface to mimic the initial adhesive steps of HIV to heparan sulfates of syndecans on target cells. This project will clarify a potentially important mechanism of immune protection by IgG in mucus secretions. The results will likely aid in developing novel active (vaccine) and passive (topical) immunization strategies against HIV that exploit the ability of IgG to trap HIV in mucus. PUBLIC HEALTH RELEVANCE: HIV readily penetrates pH neutralized mucus secretions to transmit infection. Recent evidence suggests anti- HIV antibodies protect against mucosal HIV challenges in animals and humans, yet the mechanisms of protection remain poorly understood. This project will aid in engineering active (vaccine) and passive (topical) immunization strategies that may help protect against HIV infections.